Dr Ricardo Osorio, from New York University School of Medicine in the US, who led the new research, said: "We know that about 10 to 20% of middle-aged adults in the United States have SDB and that the number jumps dramatically in those over the age of 65.

"We don't know why it becomes so prevalent but one factor may be that some of these patients are in the earliest pre-clinical stages of AD (Alzheimer's disease)."

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Signs Of Dementia

Signs Of Dementia

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A group of 68 men and women with an average age of 71 and no evidence of dementia took part in the study.

Around half (48.5%) were found to have mild SDB and a quarter had moderate-to-severe SDB.

The volunteers had spinal fluid tests and brain scans to identify any early biochemical signs of Alzheimer's disease.

Alzheimer's biomarkers were only found among lean participants with SDB. In these patients, a progressive association was seen between the severity of SDB and the level of two indicators based on proteins and brain scans.

Lean patients with SDB also tended to have a smaller hippocampus - a part of the brain crucial to memory - than other participants.

The findings were presented today at the American Thoracic Society 2013 annual conference in Philadelphia.

Dr Osorio said it was not clear which condition came first, SDB or Alzheimer's.

"It's really a chicken and egg story," he said. "Our study did not determine the direction of causality, and, in fact, didn't uncover a significant association between the two, until we broke out the data on lean and obese patients."

His team now plans to test a theory that brain damage caused by very early stage Alzheimer's can trigger SDB in lean individuals who would otherwise not be affected by the condition. Obesity is, by itself, a risk factor for breathing problems during sleep.

After an initial assessment, patients will be given continuous positive airway pressure (CPAP) treatment for sleep apnoea. After six months, they will be evaluated again for biomarker evidence of Alzheimer's.

"If the biomarkers change, it may indicate that SDB is causing AD," said Dr Osorio. "If they don't change, the probable conclusion is that these patients are going to develop AD with or without CPAP, and that AD may either be causing the apnoeas or may simply co-exist with SDB as part of ageing."

:: A second study presented at the same meeting revealed asthma to be a potential risk factor for obstructive sleep apnoea.

Researchers found that asthmatic patients were 1.70 times more likely to develop sleep apnoea over a period of eight years.